Voltage transient protection circuit for MOSFET power transistors

Abstract

A pair of MOSFET power transistors are connected in a push-pull configuration to drive a tapped transformer and a load. A pair of diodes and an energy coupling circuit containing a capacitor and resistor in parallel bias the transformer primary in such a manner as to limit transients in a nonconducting MOSFET when the other MOSFET is conducting, aided by a drop in voltage at an inductor (ferrite bead) network which develops at the moment of MOSFET transistor conduction. This is due to a momentary change in the impedance ratio.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 
     
       1. A voltage transient protection circuit for a pair of metal oxide semiconductor field effect transistors (MOSFETs) operated in a push-pull circuit with said MOSFETs each having a drain connection, a source connection, and a gate connection, including: a transformer having a tapped primary winding and a pair or primary end windings; a first of said pair of end windings connected to a first of said drain connections, and a second of said pair of end windings connected to a second of said drain connections; a pair of diodes with each of said diodes connected to a separate transformer end winding with said diodes further connected in common to a first end of an energy dissipation means; said energy dissipation means having a second end connected to said transformer tapped winding; said energy dissipation means including a parallel combination of a capacitor and a resistor; said transformer tapped winding further connected by circuit means to a source of potential for said MOSFETs and with said source of potential further including a circuit ground; second energy dissipation means having inductive means in parallel with a parallel connected diode means; said second energy dissipation means connecting said tapped primary winding to said source of potential; and both of said MOSFETs source connections being connected to said circuit ground; said diodes and said energy dissipation means limiting voltage transients in said protection circuit to a level that is not harmful to said MOSFETs. 
     
     
       2. A voltage transient protection circuit as disclosed in claim 1 wherein said source of voltage is coupled to said circuit ground with a further capacitor. 
     
     
       3. A voltage transient protection circuit as disclosed in claim 1 wherein said inductive means is a ferrite bead; and said parallel connected diode means is a parallel combination of a diode and a resistor. 
     
     
       4. A voltage transient protection circuit as disclosed in claim 3 wherein said first energy dissipation means includes a capacitor. 
     
     
       5. A voltage transient protection circuit as disclosed in claim 4 wherein said first energy dissipation means further includes a resistance connected in parallel circuit with said capacitor of said first energy dissipation means.